Long-term shifts in the growth and maturation size of Miyabe charr Salvelinus malma miyabei

Overfishing can affect life history traits, resulting in population collapse and oftentimes a decrease in length-at-age and maturation size in fish populations. However, little is known about the recovery mechanisms and time scales of these traits in exploited wild populations. In the study reported here, we documented long-term shifts in growth and mature size in Miyabe charr Salvelinus malma miyabei associated with a history of recreational fishing in Lake Shikaribetsu for approximately 80 years. Downsizing in the charr was observed when the charr population collapsed due to intensive recreational fishing. However, subsequent moratoriums and the introduction of fishing regulations, especially the implementation of a catch-and-release policy, during the following 10-30 years facilitated the recovery of population size, length-at-age and mature fish size. This study provides important insights into the biological changes and required recovery time scales of a heavily harvested population and supports management and conservation strategies.北海道然別湖に生息するミヤベイワナの成長解析を行い、約80年にわたる漁獲圧と成長や成熟魚サイズの変化の関係について検討した。その結果、過剰な遊漁による漁獲圧の上昇により、ミヤベイワナの成長が鈍化し成熟魚が小型化したと考えられた。しかし、禁漁や遊漁規制の導入により、現在は乱獲状態にあった時期と比べて資源が回復し、約10年から30年で個体の成長と成熟魚サイズが回復したことが示唆された

Relationship between temperature and embryonic period of Arabesque greenling Pleurogrammus azonus

To clarify the relationship between temperature and embryonic period of Arabesque greenling Pleurogrammus azonus, we incubated eggs at the water‑temperature conditions which approximate the range during the spawning season. The relationship between the mean rearing temperature (T; °C) and the mean embryonic period (D; days) was as follows: D = 170.63 - 20.66T + 0.85T^2

Effect of egg size on the growth rate and survival of wild walleye pollock Gadus chalcogrammus larvae

To clarify the survival process of walleye pollock larvae of the Japan Pacific stock in relation to the individual egg size, we estimated the original relative egg diameter from the diameter of the hatch check formed on larval sagittal otoliths. Significant positive correlations were obtained between egg diameter and embryonic body length, yolk sac volume, and lapillar otolith diameter in 3-year groups (2013-2015 year classes) collected with a plankton net in Funka Bay. Embryos before hatching were without a check; however, checks were observed on all larval otoliths just after hatching. The check diameters showed significant positive regressions to age at collection (days after hatching) in 2014 and 2015 year classes, however not in the 2013 year class. Diameters of eggs collected with a ring net in Funka Bay gradually increased from 2013 to 2015 year classes. During the 2013-2015 spawning seasons, the proportion of adult females of the 2007 and 2009 year classes, which had relatively high abundances, gradually increased to higher proportions with age (43%, 57%, and <73%, respectively), indicating that the mechanism by which the eggs gradually became larger. The low water temperature experienced by the spawners ca. 2 months earlier, advancement in spawner age, and increase of repeat spawners (at least twice in their life time) would contribute to the increase in egg size and growth and survival rate of walleye pollock larvae